"Reprobate" describes the condition of humans under judgment for their
rebellion against God; it says nothing about the noetic effects sin. It is
not used in Rom 1.
OTOH, Rom 1:18-20 does speak to the noetic state of unbelievers. First, in
v. 18, Paul claims that men "suppress the truth by their wickedness." This
supression may be subconscious (or unconscious--I'm never sure of the
proper psychological description of this all), in that I doubt that very
many thoughtful atheists consciously think, "Oh, I see the truth of God,
but if I were to acknowledge it, I'd have to change both my thinking and my
living, so I will supress it." But it is obvious that the truth must be
present to them and in them for them to suppress it.
Then, v. 19 says that "God has made the truth plain to them." If God makes
a truth plain then it is plain. Far from supporting the view that
unbelievers cannnot discern the truth of God because of the noetic effects
of sin, this seems to affirm that they can and do, but then reject or
Finally, v. 20 concludes that "they are without excuse." So while the
knowledge of God available from natural revelation may not be sufficient
for salvaton, it is certainly sufficient for condemnation. The contents of
natural revelation in this context are "God's invisible qualities--his
eternal power and divine nature" (NIV). These aspects of God's nature have
been visible "since the creation," which certainly would include humanity
since the Fall.
So I don't find at all compelling any argument which says that unbelievers
cannot by nature (because of the noetic effects of sin) "obtain right
knowledge of God as revealed through nature," as you put it. Rather, they
are by nature (because of the moral effects of sin) incapable of responding
in faith to that revelation, unless the Holy Spirit acts in them to "draw"
them to God, and instead supress the truth they do obtain.
>Do you really want to classify Paul's appeal to an alter to an unknown god as
>Natural theology? If so. why so?
No, I used it as an example of how theistic belief might serve as a stage
on the path to Christian belief.